An exposure of the Mather Gorge Formation, downstream of the &quot;emergency exit&quot; trail off Billy Goat Trail loop A, and upstream of the first sandy beach. Immediately upstream of this outcrop is a prominent belt of amphibolite, and there are large angular amphibolite boudin blocks embedded in this strange rock. Interpreted as migmatitic metasediments, the dark schisty layers are interpreted as relict bedding, ancient mud layers recrystallized to schist and then boudinaged (broken into pieces) due to tectonic flattening. Interesting metamorphic textures with a &quot;spongy&quot; texture can be observed in several places, as well as migmatization (partial melting) as evidenced by the felsic, coarse-grained material seen filling the boudin necks. There is chocoalte tablet boudinage at this location too (though not completely shown by this gigapan).

Also in evidence are potholes and flood-smashed trees, showing the effects of the Potomac River on these outcrops. Here in Mather Gorge, the Potomac floods relatively frequently, cleaning off these rocks and allowing us a view into their mineralogy and structure that would otherwise be obscured by lichens or river slime.